1. Field of the Invention
The present invention relates generally to printed circuit boards, and more particularly to stencils for applying solder paste therethrough onto printed circuit boards.
2. Disclosure Information
In FIGS. 1A-D and 2, a stencil 70 for applying solder paste 72 atop a printed circuit board (PCB) substrate 30 is illustrated. The stencil is typically a metallic sheet or foil with small apertures 74 defined therein, wherein the apertures are sized and arranged so as to correspond with the copper mounting pads 76 on the substrate. The steps for using the stencil involve: (1) placing the stencil 70 on the substrate 30 such that the apertures 74 are registered atop their respective mounting pads 76 (FIGS. 1A-B), (2) applying solder paste 72 to the top or "squeegee" side 78 of the stencil (FIG. 1B), (3) moving a squeegee blade 82 across the top side 78 so as to force the solder paste 72 into the apertures 74 (FIG. 1C), and (4) lowering the PCB away from the stencil, whereupon the solder mass 84 within each aperture is left atop its respective mounting pad 76 (FIG. 1D). Alternatively, a compression printing head may be used instead of a squeegee, as described in U.S. Pat. No. 5,565,033 to Gaynes et al. (incorporated herein by reference).
The shape of each stencil aperture 74 is typically the same as that of the respective mounting pad 76, and the size of the aperture is typically the same as that of the pad or smaller. For example, a 60.times.90-mil rectangular pad would typically have a correspondingly rectangular stencil aperture sized 60.times.90 mils or less (e.g., 50.times.80 mils). The aperture is typically formed with side walls 86 which are substantially straight and generally orthogonal to the top and bottom stencil surfaces 78/80, as illustrated by the apertures 74 labeled "Straight" in FIGS. 1A and 2. Alternatively, the stencil may be a "stepped stencil" wherein at least some of the apertures 74 have the configuration illustrated by the apertures labeled "Stepped" in FIGS. 1A and 2. Such stepped apertures typically comprise (1) a top surface cavity 88 having a given width/length/diameter A and a given depth B, and (2) a bottom surface cavity 90 having a given depth C and a given width/length/diameter D, wherein the cavities 88/90 are generally concentric with one another and in open communication with each other. Note that such aperture configurations provide a ledge 92 which bounds the side walls of the bottom surface cavity 90 and which is defined as the non-cavity portion of the stencil underneath the top surface cavity 88. Examples of prior art stepped stencils are provided in U.S. Pat. No. 4,872,261 to Sanyal et al. and U.S. Pat. No. 5,681,387 to Schmidt (both incorporated herein by reference).
Stepped stencils are most often used where the thickness of solder paste 72 to be deposited on the mounting pads 76 varies from one set of pads to another. By using a stepped stencil, those pads 76 whose apertures 74 are "straight" will have a thickness of solder paste generally equal to the whole thickness T of the stencil 70, while those pads whose apertures are "stepped" will have a solder paste thickness ideally equal to the depth C of the bottom surface cavity 90, as illustrated by the paste deposition labeled "Ideal" in FIG. 1D. But, in practice it is often the case that when the PCB is lowered away from the stencil, the deposition left on the pads is significantly greater or less than the "Ideal" case, as illustrated by the depositions labeled "Actual" in FIG. 1D. Here, either some of the paste that was in the top cavity 88 gets pulled away and sticks to the paste in the bottom cavity 90, thus ending up on the mounting pad, or else some of the paste in the bottom cavity 90 sticks to and stays with the paste in the top cavity 88. This presents a process control problem--namely, the amount of solder deposition becomes too variable from one deposition to another on the same PCB, and too variable for any given stepped aperture from one PCB to another.
While stepped stencils are useful for depositing different depths of solder paste in the same squeegee pass or compression head pass, it would be desirable to find a way of providing this same benefit without the variability inherent in conventional stepped stencils.